This invention is generally related to a method and system for recovering gas from subterranean gas hydrate formations. More particularly, this invention relates to a method and system for producing methane gas sequestered within subterranean methane hydrates.
A gas hydrate is a crystalline solid that is a cage-like lattice of a mechanical intermingling of gas molecules in combination with molecules of water. The name for the parent class of compounds is “clathrates” which comes from the Latin word meaning “to enclose with bars.” The structure is similar to ice but exists at temperatures well above the freezing point of ice. Gas hydrates include carbon dioxide, hydrogen sulfide, and several low carbon number hydrocarbons, including methane. Of primary interest for this invention is the recovery of methane from subterranean methane hydrates.
Methane hydrates are known to exist in large quantities in two types of geologic formations: (1) in permafrost regions where cold temperatures exist in shallow sediments and (2) beneath the ocean floor at water depths greater than 500 meters where high pressures prevail. Large deposits of methane hydrates have been located in the United States in Alaska, the west coast from California to Washington, the east coast in water depths of 800 meters, and in the Gulf of Mexico (other well know areas include Japan, Canada, and Russia).
A U.S. Geological Survey study estimates that in-place gas resources within gas hydrates consist of about 200,000 trillion cubic feet which dwarfs the previously estimated 1,400 trillion cubic feet of conventional recoverable gas reserves in the United States. Worldwide, estimates of the natural gas potential of gas hydrates approach 400 million trillion cubic feet.
Natural gas is an important energy source in the United States. It is estimated that by 2025 natural gas consumption in the United States will be nearly 31 trillion cubic feet. Given the importance and demand for natural gas the development of new cost-effective sources can be a significant benefit for American consumers.
Notwithstanding the obvious advantages and potential of methane hydrates, production of methane from gas hydrates is a challenge for the industry. When trying to extract methane from a gas hydrate the sequestered gas molecules must first be dissociated, in situ, from the hydrate. There are typically three methods known that can be used to create this dissociation.
One method is to heat the gas hydrate formation to liberate the methane molecules. This method is disclosed in U.S. Patent Application Publication No. US 2006/0032637 entitled “Method for Exploitation of Gas Hydrates” published on Feb. 16, 2006, and of common assignment with the subject application. The disclosure of this publication is incorporated herein by reference as background information with respect to the subject invention.
Another method envisioned for producing methane hydrates is to inject chemicals into the hydrate formation to change the phase behavior of the formation.
A third technique, which is the subject of the instant invention, is regarded as a depressurization method. This method involves depressurization of a gas hydrate formation and maintaining a relatively constant depressurization on the hydrate formation to allow dissociation and then withdrawing dissociated gas and water through a well casing.